Light-emitting diodes (LEDs) are the most efficient among currently available light sources. In particular, white LEDs find a rapidly expanding share in the market as the next-generation light source to replace incandescent lamps, fluorescent lamps, cold cathode fluorescent lamps (CCFL), and halogen lamps. The white LEDs are arrived at by combining a blue LED with a phosphor capable of emission upon blue light excitation. In the current mainstream industry, yellow light-emitting phosphors are combined with blue LEDs to produce pseudo-white light. Examples of suitable yellow light-emitting phosphors include Y3AlO12:Ce, (Y,Gd)3 (Al,Ga)5O12:Ce, (Y,Gd)3Al5O12:Ce, Tb3Al5O12:Ce, CaGa2S4:Eu, (Sr,Ca,Ba)2SiO4:Eu, and Ca-α-SiAlON:Eu.
JP 3700502 discloses a method for preparing a phosphor by dissolving rare earth elements Y, Gd, and Ce in stoichiometric proportions in an acid, co-precipitating the solution with oxalic acid, firing the co-precipitate to obtain an oxide of the co-precipitate, mixing it with aluminum oxide, and adding ammonium fluoride as flux thereto. The mixture is placed in a crucible and fired in air at 1,400° C. for 3 hours. The fired material is wet milled in a ball mill, washed, separated, dried, and finally sieved.
When a phosphor is synthesized by such a conventional method of particle mixing and solid-phase reaction, individual particles are single crystal particles having a smooth crystal face because the particle size is controlled by crystal growth in the flux. For this reason, part of incident light is reflected, which becomes an inhibitory factor against further improvement in absorptance. On use, the phosphor is mixed with an encapsulating resin, and a layer of the phosphor-dispersed resin is formed on blue LED. Due to a difference in thermal expansion between the phosphor and the encapsulating resin, a material interface stress is generated to gradually detract from bond strength. This results in interfacial separation between the phosphor and the encapsulating resin, creating a gas phase at the interface, which causes to reduce the efficiency of light extraction from the phosphor.
Further, in the conventional solid-phase method, a large amount of flux component is mixed for grain growth purpose. This raises a problem that some flux component is left even after conversion of the source powder into a phosphor and washing away of the flux component. When an LED device is prepared by mixing the phosphor with a resin and molding the resin on a LED chip which generates exciting light, the residual flux component becomes a factor of reducing the efficiency of extraction of phosphor emission outside the LED device because the flux component is not a fraction that is emissive in response to excitation light.